Watchcases having their top formed of a flat sapphire crystal to mask the body of the bases were proposed in Swiss patent application 2859/81. In those cases, the sockets enabling the crystal to be attached to the watch body are integrated with the crystal by means of a frame glued under the crystal.
An object of this invention, among others, is to simplify that construction without sacrificing the possibilities of decorating the case, and even increasing the strength of the crystal.
By embedding the head of the socket in the crystal itself, it is no longer necessary to fasten a frame under the crystal. Furthermore, by using two sapphire plates glued to each other, the crystal is stronger than if it were made of one piece. In addition, the two sapphire plates glued to each other provide the same possibilities of decorating the case as offered by the frame glued under the crystal.
The cones formed in the base plate of the crystal and the conical heads of the crystal fastening sockets enable the sockets to be perfectly centered by placing the heads in their respective conical seats. The pressure exerted on the base plate of the crystal is not dangerous. If, by chance, a break or crack of that base plate should occur, a piece of the latter could not come loose; it would remain glued, in fact, to the sapphire plate glued to the base plate.
The two plates comprising the crystal have the same shape. Since these two plates are glued directly to each other in the entire transparent part of the crystal, it is unnessary to be greatly concerned about the adherence of the metal coating formed on the other side of the glued plate of the crystal. Thus wholly captured between two sapphire plates, this coating is completely inaccessible; it is perfectly sheltered from all contact with foreign bodies. Barring destruction of the crystal, nothing can then alter it.
Decorative elements can also be provided in the external face of the crystal, as long as their hardness protects them from damage.
Although a sapphire plate with a simply ground face is not transparent, but only translucent, the glue has the effect of filling the fine grinding striations, so that the set of two sapphire plates glued to each other becomes perfectly transparent. The costly operation of polishing one of the faces of the two sapphire plates is thus avoided. Furthermore, such a crystal is anti-reflecting. It is then no longer necessary to subject the face of the dial to special treatment intended to make it dull. Finally, this crystal makes it possible to produce, as desired, dull or bright metal coatings, on the other side of the glued plate, depending on whether the simply ground face in question is the lower plate of the glued sapphire plate or the upper face of the base plate of the crystal.
An independent frame of precious or semiprecious stone, for example, can also be inserted between the two sapphire plates glued to each other, as long as its thickness preferably does not exceed two-tenths of a millimeter.
In the case of quadrangular (rectangular or square) crystals, it is not necessary to glue a single plate on the base plate of the crystal. It is possible to prepare one plate extending at least into the transparent area of the crystal and opaque plates for masking the crystal fastening sockets, while forming the decoration of the visible face of the case, those different plates being perfectly juxtaposed, so as to cover the whole face of the base plate of the crystal while being glued to the latter.
As the set forming the crystal is relatively thick (which does not increase the apparent thickness of the watch, by reason of the transparency of the crystal), it is possible to hollow out rather deep seats in order to accommodate the cut diamond culet there and in which the latter can be glued, avoiding the risk of losing those stones.